GH-152: Inject nullChannel for confirm acks

Fixes #152

Reduce log noise since we only express interest in nacks.

spring-cloud-stream-binder-rabbit/src/main/java/org/springframework/cloud/stream/binder/rabbit/RabbitMessageChannelBinder.java

@@ -285,6 +285,8 @@ protected MessageHandler createProducerMessageHandler(final ProducerDestination
 			checkConnectionFactoryIsErrorCapable();
 			endpoint.setReturnChannel(errorChannel);
 			endpoint.setConfirmNackChannel(errorChannel);
+			endpoint.setConfirmAckChannel(getApplicationContext().getBean(
+					IntegrationContextUtils.NULL_CHANNEL_BEAN_NAME, MessageChannel.class));
 			endpoint.setConfirmCorrelationExpressionString("#root");
 			endpoint.setErrorMessageStrategy(new DefaultErrorMessageStrategy());
 		}